Structural analysis of deeply charged Li(Ni0.95Co0.04Al0.01)O2 cathode for Li-ion battery.

LiNi0.95Co0.04Al0.01O2 (NCA95) is charged up to 4.6 V to study its structural stability at a highly delithiated state using transmission electron microscopy (TEM). The TEM analysis shows that the localized depletion of Li ions near the surface triggers the transition from the H3 phase to the H4 phase with the H4 phase with the O1 stacking appearing as a series of stacking faults even at 4.4 V. The H3 → H4 transition appears to be irreversible and leads to the initial capacity loss. In addition, intraparticle cracks are observed when charged above 4.3 V. These intraparticle microcracks, unlike interparticle cracks that become sealed upon Li-uptake, likely remain during deintercalation, compromising the mechanical stability of the cathode and lead to fast deterioration of the cycling stability. The TEM analysis of the overcharged NCA95 cathode suggests a clear limit above which the cathode can be cycled without significant capacity loss. The introduction of doping elements that promote the migration of Ni2+ ions into the Li layer would hinder the H3 → H4 transition and help suppress the intraparticle cracks.